1. Field of the Invention
The present invention relates to circuits that mitigate signal distortion caused by a variable complex impedance between a connector on a semiconductor die and one or more microspring or anisotropic-film inter-component connectors.
2. Related Art
As integrated-circuit (IC) technology continues to scale to smaller critical dimensions, it is increasingly difficult for existing inter-chip connections to provide suitable communication characteristics, such as: high bandwidth, low power, reliability and low cost. Several technologies have been proposed to address this problem. These proposed technologies include: proximity communication or PxC (for example, with capacitive inter-chip contacts), inter-chip microsprings (with conductive inter-chip contacts), anisotropic films (for example, where the anisotropic film includes an elastomer), and a combination of PxC with microsprings (with capacitive inter-chip contacts). However, the proposed techniques often introduce additional packaging and reliability challenges.
PxC based on capacitive inter-chip contacts provides dense inter-chip connections, with a pitch between neighboring pads on the order of 10-100 μm. However, PxC typically requires a similar order of mechanical alignment. It can be difficult to maintain this alignment in the presence of vibrations and thermal stress using a low-cost chip package. Furthermore, the capacitance of the inter-chip contacts can be small, which makes it challenging to couple high-capacity power supplies using PxC.
Microsprings can be fabricated on a wide variety of surfaces, including: printed circuit boards (PCBs), organic or ceramic IC packages or on the surface of ICs themselves. They can be fabricated with an areal density of inter-chip connections that exceeds the density of input/output (I/O) signals on high performance ICs, and can provide electrical contacts without the use of solder. Moreover, microsprings can be designed to have more compliance than is possible by using PxC alone, which increases the tolerance to mechanical movement and misalignment. However, microsprings are typically required to make and maintain conductive contacts with connectors on ICs. In order to achieve such conductive contacts, the microsprings typically have sharp tips that can scrape through any oxide or passivation layers above the connectors on the ICs during a scrub-in process, which increases the fabrication costs of the microsprings. Furthermore, conductive contacts are often achieved by increasing the force between the microsprings and the connectors on an IC in a chip package, which also increases cost. In addition, the sharp tips and large forces can produce foreign particles (such as debris) that can reduce the conductivity of contacts over time, thereby reducing reliability and limiting the number of mating cycles.
Anisotropic conductive films can be fabricated by introducing conductive elements into an insulating elastic film so that the conductive elements generally line up normal to the surface of the film. Then, by placing the anisotropic film against a chip pad and compressing it, the conductive elements can make conductive contact, while the non-conductive film maintains isolation among neighboring chip pads. Unlike the microsprings, conduction through the anisotropic film typically involves conduction between the chip pad and its proximal conductive elements in the anisotropic film, and among the various conductive elements that are adjacent to each other within the anisotropic film. Similar to microsprings, anisotropic films often suffer from reliability issues due to the potential for the conductive elements to fail to make adequate contact with each other and with the chip pad. While reliability can be increased by increasing the compressive force, the chip package typically has to provide and maintain this higher force. In general, higher forces within a chip package decrease the chip-package reliability in other ways and increase the packaging cost.
In order to overcome such scrub-in and reliability problems, inter-chip connections that combine PxC and microsprings or anisotropic films have been proposed. However, this approach introduces additional challenges. For example, the capacitive (or inductive) signaling techniques used in PxC typically do not tolerate conductive inter-chip contacts. As a consequence, the oxide layer above the connectors on the ICs needs to be thick and hard enough to prevent cracking, which can cause conductive contacts. This thicker oxide layer reduces the energy that is capacitively coupled between chips, which makes receiving electrical signals more difficult. In addition, it limits the amount of power that can be supplied to an IC via capacitively coupled microsprings or anisotropic films.
Hence, what is needed is a technique for achieving inter-chip connections without the problems described above.